Nitride-based semiconductor device and method of fabricating the same

ABSTRACT

A method of fabricating a nitride-based semiconductor device capable of reducing contact resistance between a nitrogen face of a nitride-based semiconductor substrate or the like and an electrode is provided. This method of fabricating a nitride-based semiconductor device comprises steps of etching the back surface of a first semiconductor layer consisting of either an n-type nitride-based semiconductor layer or a nitride-based semiconductor substrate having a wurtzite structure and thereafter forming an n-side electrode on the etched back surface of the first semiconductor layer.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a nitride-based semiconductordevice and a method of fabricating the same, and more particularly, itrelates to a nitride-based semiconductor device having an electrode anda method of fabricating the same.

[0003] 2. Description of the Background Art

[0004] A nitride-based semiconductor laser device has recently beenexpected as a light source for an advanced large capacity optical disk,and actively developed.

[0005] In general, an insulating sapphire substrate is employed forforming a nitride-based semiconductor laser device. When a nitride-basedsemiconductor layer is formed on the sapphire substrate, however, alarge number of defects (dislocations) are disadvantageously formed inthe nitride-based semiconductor layer due to large difference betweenthe lattice constants of the sapphire substrate and the nitride-basedsemiconductor layer. Consequently, the characteristics of thenitride-based semiconductor laser device are disadvantageously reduced.

[0006] In this regard, a nitride-based semiconductor laser deviceemploying a nitride-based semiconductor substrate such as a GaNsubstrate having small difference in lattice constant with respect to anitride-based semiconductor layer is proposed in general.

[0007]FIG. 7 is a sectional view showing a conventional nitride-basedsemiconductor laser device employing an n-type GaN substrate 101.Referring to FIG. 7, nitride-based semiconductor layers (102 to 110) aregrown on a Ga face ((HKLM) plane: M denotes a positive integer) to beimproved in crystallinity in a process of fabricating the conventionalnitride-based semiconductor laser device. A nitrogen face ((HKL-M)plane: M denotes a positive integer) of the n-type GaN substrate 101having a wurtzite structure is employed as the back surface, so that ann-side electrode 112 is formed on this back surface of the n-type GaNsubstrate 101. The fabrication process for the conventionalnitride-based semiconductor laser device is now described in detail.

[0008] As shown in FIG. 7, an n-type layer 102 consisting of n-type GaNhaving a thickness of about 3 μm, an n-type buffer layer 103 consistingof n-type In_(0.05)Ga_(0.95)N having a thickness of about 100 nm, ann-type cladding layer 104 consisting of n-type Al_(0.05)Ga_(0.95)Nhaving a thickness of about 400 nm, an n-type light guide layer 105consisting of n-type GaN having a thickness of about 70 nm, an MQW(multiple quantum well) active layer 106 having an MQW structure, ap-type layer 107 consisting of p-type Al_(0.2)Ga_(0.8)N having athickness of about 200 nm, a p-type light guide layer 108 consisting ofp-type GaN having a thickness of about 70 nm, a p-type cladding layer109 consisting of p-type Al_(0.05)Ga_(0.95)N having a thickness of about400 nm and a p-type contact layer 110 consisting of p-type GaN having athickness of about 100 nm are successively formed on the upper surface(Ga face) of the n-type GaN substrate 101 having a thickness of about300 μm to about 500 μm.

[0009] Then, a p-side electrode 111 is formed on a prescribed region ofthe upper surface of the p-type contact layer 110. The back surface ofthe n-type GaN substrate 101 is polished until the thickness of then-type GaN substrate 101 reaches a prescribed level of about 100 μm, andan n-side electrode 112 is thereafter formed on the back surface(nitrogen face) of the n-type GaN substrate 101. Finally, the n-type GaNsubstrate 101 and the layers 102 to 110 are cleft thereby performingelement isolation and forming a cavity facet. Thus, the conventionalnitride-based semiconductor laser device shown in FIG. 7 is completed.

[0010] In the conventional nitride-based semiconductor laser deviceshown in FIG. 7, however, the n-type GaN substrate 101 is so hard thatit is difficult to excellently perform device isolation and form thecavity facet by cleavage. In order to cope with such inconvenience, amethod of mechanically polishing the back surface of the n-type GaNsubstrate 101 before the cleavage step for reducing irregularity on theback surface thereby excellently performing element isolation andforming the cavity facet is proposed. This method is disclosed inJapanese Patent Laying-Open No. 2002-26438, for example.

[0011] In the aforementioned conventional method disclosed in JapanesePatent Laying-Open No. 2002-26438, however, stress is applied in thevicinity of the back surface of the n-type GaN substrate 101 when theback surface of the n-type GaN substrate 101 is mechanically polished.Therefore, microscopic defects such as cracks are disadvantageouslyformed in the vicinity of the back surface of the n-type GaN substrate101. Consequently, contact resistance between the n-type GaN substrate101 and the n-side electrode 112 formed on the back surface (nitrogenface) thereof is disadvantageously increased.

[0012] Further, the nitrogen face of the n-type GaN substrate 101 is soeasily oxidized that the contact resistance between the n-type GaNsubstrate 101 and the n-side electrode 112 formed on the back surface(nitrogen face) thereof is disadvantageously increased also by this.

SUMMARY OF THE INVENTION

[0013] An object of the present invention is to provide a method offabricating a nitride-based semiconductor device capable of reducingcontact resistance between the back surface of a nitride-basedsemiconductor substrate or the like and an electrode.

[0014] Another object of the present invention is to reduce the numberof defects in the vicinity of the back surface of the nitride-basedsemiconductor substrate or the like in the aforementioned method offabricating a nitride-based semiconductor device.

[0015] Still another object of the present invention is to provide anitride-based semiconductor device capable of reducing contactresistance between the back surface of a nitride-based semiconductorsubstrate or the like and an electrode.

[0016] In order to attain the aforementioned objects, a method offabricating a nitride-based semiconductor device according to a firstaspect of the present invention comprises steps of etching the backsurface of a first semiconductor layer consisting of either an n-typenitride-based semiconductor layer or a nitride-based semiconductorsubstrate having a wurtzite structure and thereafter forming an n-sideelectrode on the etched back surface of the first semiconductor layer.

[0017] In the method of fabricating a nitride-based semiconductor deviceaccording to the first aspect, the back surface of the firstsemiconductor layer consisting of either an n-type nitride-basedsemiconductor layer or a nitride-based semiconductor substrate having awurtzite structure is etched as hereinabove described, whereby a regionincluding defects in the vicinity of the back surface of the firstsemiconductor layer resulting from a polishing step or the like can beremoved for reducing the number of defects in the vicinity of the backsurface of the first semiconductor layer. Thus, an electron carrierconcentration can be inhibited from reduction resulting from trap ofelectron carriers by defects, so that the electron carrier concentrationcan be increased on the back surface of the first semiconductor layer.Consequently, contact resistance between the first semiconductor layerand the n-side electrode can be reduced. Further, the back surface ofthe first semiconductor layer is so etched that flatness thereof can beimproved as compared with that of a mechanically polished back surface.Thus, the n-side electrode formed on the back surface of the firstsemiconductor layer can also be improved in flatness, whereby adhesionbetween the n-side electrode and a radiator base can be improved whenthe former is mounted on the latter. Consequently, excellent radiabilitycan be attained. Further, the n-side electrode formed on the backsurface of the first semiconductor layer can be so improved in flatnessthat wire bondability with respect to the n-side electrode can beimproved when the n-side electrode is wire-bonded.

[0018] In the aforementioned method of fabricating a nitride-basedsemiconductor device according to the first aspect, the back surface ofthe first semiconductor layer preferably includes a nitrogen face of thefirst semiconductor layer. The term “nitrogen face” denotes a wideconcept indicating not only a complete nitrogen face but also a surfacemainly formed by a nitrogen face. More specifically, the term “nitrogenface” includes a surface having a nitrogen face of at least 50% in thepresent invention. When formed by a nitrogen face, the back surface ofthe first semiconductor layer is so easily oxidized that the oxidizedportion of the back surface can be removed by etching. Thus, contactresistance between the first semiconductor layer and the n-sideelectrode can be further reduced.

[0019] In the aforementioned method of fabricating a nitride-basedsemiconductor device according to the first aspect, the etching steppreferably includes a step of etching the back surface of the firstsemiconductor layer by dry etching. According to this structure, theback surface of the first semiconductor layer can be easily improved inflatness and the number of defects can be reduced in the vicinity of theback surface due to the dry etching.

[0020] In the aforementioned method of fabricating a nitride-basedsemiconductor device including the step of etching the back surface ofthe first semiconductor layer by dry etching, the step of etching theback surface of the first semiconductor layer by dry etching preferablyincludes a step of etching the back surface of the first semiconductorlayer by reactive ion etching with Cl₂ gas and BCl₃ gas. According tothis structure, the back surface of the first semiconductor layer can beeasily improved in flatness and the number of defects can be easilyreduced in the vicinity of the back surface. In this case, the ratio ofthe flow rate of BCl₃ gas to the flow rate of Cl₂ gas in the step ofetching the back surface of the first semiconductor layer by thereactive ion etching is preferably at least 30% and not more than 70%.It has been experimentally confirmed that the back surface of the firstsemiconductor layer can be improved in flatness in this range of theratio of the flow rate of BCl₃ gas to that of Cl₂ gas, and hence theback surface of the first semiconductor layer can be reliably improvedin flatness by setting the ratio within this range.

[0021] In the aforementioned method of fabricating a nitride-basedsemiconductor device including the step of etching the back surface ofthe first semiconductor layer by dry etching, the etching depth and theetching time in the step of etching the back surface of the firstsemiconductor layer by dry etching are preferably proportional to eachother. According to this structure, the etching depth can be accuratelycontrolled by adjusting the etching time.

[0022] In the aforementioned method of fabricating a nitride-basedsemiconductor device according to the first aspect, the etching steppreferably includes a step of etching the back surface of the firstsemiconductor layer thereby converting the back surface of the firstsemiconductor layer to a mirror surface. According to this structure,the back surface of the first semiconductor layer can be furtherimproved in flatness.

[0023] The aforementioned method of fabricating a nitride-basedsemiconductor device according to the first aspect preferably furthercomprises a step of performing heat treatment after the step of formingthe n-side electrode. According to this structure, contact resistancebetween the first semiconductor layer and the n-side electrode can befurther reduced.

[0024] In the aforementioned method of fabricating a nitride-basedsemiconductor device according to the first aspect, the etching steppreferably includes a step of etching the back surface of the firstsemiconductor layer by a thickness of at least about 1 μm. According tothis structure, a region including defects in the vicinity of the backsurface of the first semiconductor layer resulting from a polishing stepor the like can be so sufficiently removed that the number of defectscan be further reduced in the vicinity of the back surface of the firstsemiconductor layer.

[0025] In the aforementioned method of fabricating a nitride-basedsemiconductor device according to the first aspect, the firstsemiconductor layer may include the n-type nitride-based semiconductorlayer or the nitride-based semiconductor substrate consisting of atleast one material selected from a group consisting of GaN, BN, AlN, InNand TlN. Further, the n-side electrode may include an Al film.

[0026] In the aforementioned method of fabricating a nitride-basedsemiconductor device according to the first aspect, the nitride-basedsemiconductor device is preferably a nitride-based semiconductorlight-emitting device. According to this structure, contact resistancebetween the first semiconductor layer and the n-side electrode can bereduced in the nitride-based semiconductor light-emitting device,whereby the nitride-based semiconductor light-emitting device can attainexcellent emissivity.

[0027] The aforementioned method of fabricating a nitride-basedsemiconductor device according to the first aspect preferably furthercomprises a step of dipping a nitrogen face of the etched firstsemiconductor layer in a solution containing at least one of chlorine,fluorine, bromine, iodine, sulfur and ammonium in advance of the step offorming the n-side electrode. According to this structure, residuesresulting from etching can be easily removed from the back surface ofthe first semiconductor layer. Thus, contact resistance between thefirst semiconductor layer and the n-side electrode can be furtherreduced. In this case, the method of fabricating a nitride-basedsemiconductor device further comprises a step of performing hydrochloricacid treatment on the back surface of the first semiconductor layer withan HCl solution in advance of the step of forming the n-side electrode.According to this structure, chlorine-based residues adhering to theback surface of the first semiconductor layer due to the etching can beeasily removed.

[0028] The aforementioned method of fabricating a nitride-basedsemiconductor device according to the first aspect preferably furthercomprises a step of polishing the back surface of the firstsemiconductor layer in advance of the etching step. Also when polishingthe back surface of the first semiconductor layer, the back surface ofthe first semiconductor layer can be improved in flatness and the numberof defects resulting from polishing can be reduced in the vicinity ofthe back surface through the etching step following the polishing step.

[0029] In the aforementioned method of fabricating a nitride-basedsemiconductor device according to the first aspect, the etching steppreferably includes a step of etching the back surface of the firstsemiconductor layer by wet etching. According to this structure, theback surface of the first semiconductor layer can be easily improved inflatness and the number of defects can be easily reduced in the vicinityof the back surface due to the wet etching. In this case, the step ofetching the back surface of the first semiconductor layer by wet etchingpreferably includes a step of etching the back surface of the firstsemiconductor layer with at least one etchant selected from a groupconsisting of aqua regia, KOH and K₂S₂O₈. Further, the step of etchingthe back surface of the first semiconductor layer by wet etchingpreferably includes a step of etching the back surface of the firstsemiconductor layer while increasing the temperature to about 120° C.According to this structure, the etching rate can be increased to about10 times that in wet etching carried out under the room temperature.

[0030] A method of fabricating a nitride-based semiconductor deviceaccording to a second aspect of the present invention comprises steps ofetching a nitrogen face of a first semiconductor layer consisting ofeither an n-type nitride-based semiconductor layer or a nitride-basedsemiconductor substrate having a wurtzite structure by dry etching andthereafter forming an n-side electrode on the etched nitrogen face ofthe first semiconductor layer.

[0031] In the method of fabricating a nitride-based semiconductor deviceaccording to the second aspect, the nitrogen face of the firstsemiconductor layer consisting of either an n-type nitride-basedsemiconductor layer or a nitride-based semiconductor substrate having awurtzite structure is etched by dry etching as hereinabove described,whereby a region including defects in the vicinity of the firstsemiconductor layer resulting from a polishing step or the like can beso reduced that the number of defects can be reduced in the vicinity ofthe nitrogen face of the first semiconductor layer. Thus, reduction ofan electron carrier concentration resulting from trap of electroncarriers by defects can be suppressed, whereby the electron carrierconcentration can be increased in the nitrogen face of the firstsemiconductor layer. Consequently, contact resistance between the firstsemiconductor layer and the n-side electrode can be reduced. Further,the nitrogen face of the first semiconductor layer is so etched by dryetching that flatness thereof can be improved as compared with that of amechanically polished nitrogen face. Thus, the n-side electrode formedon the nitrogen face of the first semiconductor layer can also beimproved in flatness, whereby adhesion between the n-side electrode anda radiator base can be improved when the former is mounted on thelatter. Consequently, high radiability can be attained. Further, then-side electrode formed on the nitrogen face of the first semiconductorlayer can be so improved in flatness that wire bondability with respectto the n-side electrode can be improved when the n-side electrode iswire-bonded.

[0032] A nitride-based semiconductor device according to a third aspectof the present invention is formed through steps of etching the backsurface of a first semiconductor layer consisting of either an n-typenitride-based semiconductor layer or a nitride-based semiconductorsubstrate having a wurtzite structure and thereafter forming an n-sideelectrode on the etched back surface of the first semiconductor layer.

[0033] In the nitride-based semiconductor device according to the thirdaspect, a region including defects in the vicinity of the firstsemiconductor layer resulting from a polishing step or the like can beremoved by etching the back surface of the first semiconductor layerconsisting of either an n-type nitride-based semiconductor layer or anitride-based semiconductor substrate having a wurtzite structure ashereinabove described, whereby the number of defects can be reduced inthe vicinity of the back surface of the first semiconductor layer. Thus,an electron carrier concentration can be inhibited from reductionresulting from trap of electron carriers by defects, whereby theelectron carrier concentration can be increased on the back surface ofthe first semiconductor layer. Consequently, contact resistance betweenthe first semiconductor layer and the n-side electrode can be reduced.Further, the back surface of the first semiconductor layer is so etchedthat flatness thereof can be improved as compared with that of amechanically polished back surface. Thus, the n-side electrode formedthe back surface of the first semiconductor layer can also be improvedflatness, whereby adhesion between the n-side electrode and a radiatorbase can be improved when the former is mounted on the latter. Further,the n-side electrode formed on the back surface of the firstsemiconductor layer can be so improved in flatness that wire bondabilitywith respect to the n-side electrode can be improved when the n-sideelectrode is wire-bonded.

[0034] A nitride-based semiconductor device according to a fourth aspectof the present invention comprises a first semiconductor layerconsisting of either an n-type nitride-based semiconductor layer or anitride-based semiconductor substrate having a wurtzite structure and ann-side electrode formed on the back surface of the first semiconductorlayer, while contact resistance between the n-side electrode and thefirst semiconductor layer is not more than 0.05 Ωcm².

[0035] In the nitride-based semiconductor device according to the fourthaspect, the contact resistance between the n-side electrode and thefirst semiconductor layer is set to not more than 0.05 Ωcm², so that thenitride-based semiconductor device can attain excellent devicecharacteristics by reducing the contact resistance between the n-sideelectrode and the first semiconductor layer.

[0036] In the aforementioned nitride-based semiconductor deviceaccording to the fourth aspect, an electron carrier concentration ispreferably at least 1×10¹⁷ cm⁻³ in the vicinity of the interface betweenthe first semiconductor layer and the n-side electrode. According tothis structure, the nitride-based semiconductor device can easily reducethe contact resistance between the n-side electrode and the firstsemiconductor layer.

[0037] In the aforementioned nitride-based semiconductor deviceaccording to the fourth aspect, a dislocation density is preferably notmore than 1×10⁹ cm⁻² in the vicinity of the interface between the firstsemiconductor layer and the n-side electrode. According to thisstructure, the number of defects (dislocations) can be reduced in thevicinity of the interface between the first semiconductor layer and then-side electrode, whereby the contact resistance can be reduced in theinterface between the first semiconductor layer and the n-sideelectrode.

[0038] In the aforementioned nitride-based semiconductor deviceaccording to the fourth aspect, the back surface of the firstsemiconductor layer preferably includes a nitrogen face of the firstsemiconductor layer.

[0039] In the aforementioned nitride-based semiconductor deviceaccording to the fourth aspect, the first semiconductor layer mayinclude the n-type nitride-based semiconductor layer or thenitride-based semiconductor substrate consisting of at least onematerial selected from a group consisting of GaN, BN, AlN, InN and TlN.Further, the n-side electrode may include an Al film.

[0040] In the aforementioned nitride-based semiconductor deviceaccording to the fourth aspect, the nitride-based semiconductor deviceis preferably a nitride-based semiconductor light-emitting device.According to this structure, contact resistance between the firstsemiconductor layer and the n-side electrode can be reduced in thenitride-based semiconductor light-emitting device, whereby thenitride-based semiconductor light-emitting device can attain excellentemissivity.

[0041] The foregoing and other objects, features, aspects and advantagesof the present invention will become more apparent from the followingdetailed description of the present invention when taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] FIGS. 1 to 3 are sectional views for illustrating a process offabricating a nitride-based semiconductor laser device according to anembodiment of the present invention;

[0043]FIG. 4 is an enlarged sectional view in the step shown in FIG. 3;

[0044]FIG. 5 is a perspective view for illustrating the process offabricating a nitride-based semiconductor laser device according to theembodiment of the present invention;

[0045]FIG. 6 is a graph showing change of an etching rate in a case ofvarying the etching gas ratio in RIE; and

[0046]FIG. 7 is a sectional view showing a conventional nitride-basedsemiconductor laser device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0047] An embodiment of the present invention is now described withreference to the drawings.

[0048] A process of fabricating a nitride-based semiconductor laserdevice according to the embodiment is described with reference to FIGS.1 to 5. According to this embodiment, an oxygen-doped n-type GaNsubstrate 1 having a wurtzite structure is formed by a method disclosedin Japanese Patent Laying-Open No. 2000-44400, for example. Morespecifically, an oxygen-doped n-type GaN layer is formed on a GaAssubstrate (not shown) by HVPE with a thickness of about 120 μm to about400 μm. Thereafter the GaAs substrate is removed thereby obtaining then-type GaN substrate 1 shown in FIG. 1. The n-type GaN substrate 1 has asubstrate carrier concentration of 5×10¹⁸ cm⁻³ according to Hall effectmeasurement. The impurity concentration of the n-type GaN substrate 1according to SIMS (secondary ion mass spectroscopy) analysis is 1×10¹⁹cm⁻³. The n-type GaN substrate 1 is an example of the “firstsemiconductor layer” in the present invention.

[0049] An n-type buffer layer 2 consisting of n-type GaN having athickness of about 5 μm, an n-type cladding layer 3 consisting of n-typeAl_(0.08)Ga_(0.92)N having a thickness of about 1 μm, an MQW activelayer 4 consisting of InGaN, a p-type cladding layer 5 consisting ofp-type Al_(0.08)Ga_(0.92)N having a thickness of about 0.28 μm and ap-type contact layer 6 consisting of p-type GaN having a thickness ofabout 70 nm are successively formed on the upper surface (Ga face),i.e., the (0001) plane of the n-type GaN substrate 1 by atmosphericpressure MOCVD under a pressure of about 1 atm (about 100 kPa).

[0050] The MQW active layer 4 is formed by alternately stacking fourbarrier layers of GaN each having a thickness of about 20 nm and threewell layers of In_(0.15)Ga_(0.85)N each having a thickness of about 3.5nm. Ga(CH₃)₃, In(CH₃)₃, Al(CH₃)₃ and NH₃ are employed as material gases,and H₂ and N₂ are employed as carrier gases. According to thisembodiment, the quantities of these material gases are varied foradjusting the compositions of the layers 2 to 6. SiH₄ gas (Si) isemployed as the n-type dopant for the n-type buffer layer 2 and then-type cladding layer 3. Cp₂Mg gas (Mg) is employed as the p-type dopantfor the p-type cladding layer 5 and the p-type contact layer 6.

[0051] Then, the p-type contact layer 6 and the p-type cladding layer 5are partially etched through photolithography and etching, therebyforming projecting portions (ridge portions) of about 2 μm in thicknessconsisting of projecting portions of the p-type cladding layer 5 andp-type contact layers 6, as shown in FIG. 2. Then, p-side electrodes 7consisting of Pt films having a thickness of about 1 nm, Pd films havinga thickness of about 10 nm and Ni films having a thickness of about 300nm in ascending order are formed on the upper surfaces of the p-typecontact layers 6. Thus, a nitride-based semiconductor laser devicestructure 20 is formed to include a region formed with a plurality ofelements as shown in FIG. 2.

[0052] Thereafter the back surface (nitrogen face) of the n-type GaNsubstrate 1 is mechanically polished as shown in FIGS. 3 and 4. Amechanical polisher 30 employed for this polishing step is formed by aglass plate 11 having a flat surface, a holder 12 supported to bevertically movable and rotatable along arrow R and a buff 13, as shownin FIG. 3. An abrasive (not shown) consisting of diamond, silicon oxideor alumina having particle roughness of about 0.2 μm to about 1 μm isarranged on the buff 13. This abrasive can particularly excellentlypolish the back surface of the n-type GaN substrate 1 if the particleroughness thereof is in the range of about 0.2 μm to about 0.5 μm. Thenitride-based semiconductor laser device structure 20 is mounted on thelower surface of the holder 12 at an interval through wax 14 not todirectly come into contact with the holder 12, as shown in FIGS. 3 and4. Thus, the nitride-based semiconductor laser device structure 20 isprevented from breaking in mechanical polishing. A flat polishing platemade of metal may be used instead of the glass plate 11.

[0053] The mechanical polisher 30 shown in FIG. 3 is employed forpolishing the back surface (nitrogen face) of the n-type GaN substrate 1so that the thickness of the n-type GaN substrate 1 reaches about 120 Mmto about 180 am. More specifically, the back surface (see FIG. 4) of then-type GaN substrate 1 of the nitride-based semiconductor laser devicestructure 20 mounted on the lower surface of the holder 12 is pressedagainst the upper surface of the buff 13 provided with the abrasive witha constant load. The holder 12 is rotated along arrow R while feedingwater or oil to the buff 13 (see FIG. 3). Thus, the back surface of then-type GaN substrate 1 is polished until the thickness of the n-type GaNsubstrate 1 reaches about 120 μm to about 180 μm. The n-type GaNsubstrate 1 is worked so that the thickness thereof is in the range ofabout 120 μm to about 180 μm, since a cleavage step described later canbe excellently carried out when the thickness of the n-type GaNsubstrate 1 is within this range.

[0054] According to this embodiment, the back surface (nitrogen face) ofthe n-type GaN substrate 1 is thereafter etched for about 20 minutes byreactive ion etching (RIE). This etching is carried out under conditionsof a Cl₂ gas flow rate of 10 sccm, a BCl₃ gas flow rate of 5 sccm, anetching pressure of about 3.3 Pa and RF power of 200 W (0.63 W/cm²)under the room temperature. Thus, the back surface (nitrogen face) ofthe n-type GaN substrate 1 is removed by a thickness of about 1 μm.Consequently, a region, including defects resulting from theaforementioned mechanical polishing, in the vicinity of the back surfaceof the n-type GaN substrate 1 can be removed. Further, the back surfaceof the n-type GaN substrate 1 can be worked into a flatter mirrorsurface as compared with that worked by only mechanical polishing. Themirror surface is defined as a surface state allowing excellent visualconfirmation of a reflected image of the back surface of the n-type GaNsubstrate 1.

[0055] In order to confirm the effect of the aforementioned etching, thedefect (dislocation) density on the back surface of the n-type GaNsubstrate 1 was measured before and after etching by TEM (transmissionelectron microscope) analysis. Consequently, it has been proved that thedefect density, which was at least 1×10¹⁰ cm⁻² before etching, wasreduced to below 1×10⁶ cm⁻² after the etching. Further, the electroncarrier concentration in the vicinity of the back surface of the etchedn-type GaN substrate 1 was measured with an electrochemical C-Vprofiler. Consequently, the electron carrier concentration in thevicinity of the back surface of the etched n-type GaN substrate 1 was atleast 1.0×10¹⁸ cm⁻³. Thus, it has been recognized that the electroncarrier concentration in the vicinity of the back surface can be set toa level substantially identical to the substrate carrier concentration(5×10¹⁸ cm⁻³) of the n-type GaN substrate 1.

[0056] Under the aforementioned etching conditions, the etching time andthe etching depth are proportional to each other. Therefore, the etchingdepth can be accurately controlled by adjusting the etching time. Theetching rate and the surface state vary with the composition of etchinggases. FIG. 6 is a graph showing change of the etching rate uponvariation of the etching gas ratio in RIE. In this case, the Cl₂ gasflow rate was fixed to 10 sccm and the BCl₃ gas flow rate was varied formeasuring the etching rate. Consequently, it has been proved that theetched surface is converted to a flat mirror surface when the ratio ofthe BCl₃ gas flow rate to the Cl₂ gas flow rate is in the range of atleast 30% and not more than 70%, as shown in FIG. 6. When the ratio ofthe BCl₃ gas flow rate to the Cl₂ gas flow rate was less than 5% or inexcess of 85%, the etched surface was damaged in flatness and clouded.

[0057] After the aforementioned etching step, the nitride-basedsemiconductor laser device structure 20 is dipped in an HCl solution(concentration: 10%) under the room temperature for 1 minute therebyperforming hydrochloric acid treatment. Thus, chlorine-based residuesadhering to the back surface of the n-type GaN substrate 1 in the RIEstep are removed.

[0058] Thereafter an n-side electrode 8 consisting of an Al film havinga thickness of 6 nm, an Si film having a thickness of 2 nm, an Ni filmhaving a thickness of 10 nm and an Au film having a thickness of 300 nmsuccessively from a side closer to the back surface of the n-type GaNsubstrate 1 is formed on the back surface (nitrogen face) of the n-typeGaN substrate 1 of the nitride-based semiconductor laser devicestructure 20 by sputtering or vacuum deposition.

[0059] Finally, elements are isolated and a cavity facet is formed bycleavage, thereby completing the nitride-based semiconductor laserdevice according to this embodiment as shown in FIG. 5.

[0060] In the process of fabricating a nitride-based semiconductor laserdevice according to this embodiment, the back surface (nitrogen face) ofthe n-type GaN substrate 1 is etched by RIE as hereinabove described,whereby the region, including defects resulting from the polishing step,in the vicinity of the back surface of the n-type GaN substrate 1 can beremoved. Thus, the electron carrier concentration can be inhibited fromreduction resulting from trap of electron carriers by defects. Whenformed by a nitrogen face, the back surface of the n-type GaN substrate1 is easily oxidized and hence the oxidized part can be removed byetching. Consequently, the contact resistance between the n-type GaNsubstrate 1 and the n-side electrode 8 can be reduced. Contactresistance between the n-type GaN substrate 1 and the n-side electrode 8of a nitride-based semiconductor laser device actually preparedaccording to this embodiment measured by a TLM (transmission line model)method was not more than 2.0×10⁻⁴ Ωcm². When the n-side electrode 8 wasformed on the back surface (nitrogen face) of the n-type GaN substrate 1and the structure was heat-treated in a nitrogen gas atmosphere of 500°C. for 10 minutes, the contact resistance was further reduced to1.0×10⁻⁵ Ωcm².

[0061] In the process of fabricating a nitride-based semiconductor laserdevice according to this embodiment, the back surface of the n-type GaNsubstrate 1 is etched by RIE as hereinabove described, whereby the backsurface of the n-type GaN substrate 1 can be further improved inflatness as compared with a mechanically polished back surface. Thus,the n-side electrode 8 formed on the back surface of the n-type GaNsubstrate 1 can be also improved in flatness. When the nitride-basedsemiconductor laser device is mounted in a junction-down system, wirebondability with respect to the n-side electrode 8 can be improved. Whenthe n-side electrode 8 is mounted on a radiator base (submount),adhesion between the n-side electrode 8 and the radiator base can beimproved for attaining excellent radiability.

[0062] In order to confirm the effect of the present invention etchingthe back surface (nitrogen face) of the n-type GaN substrate 1 by RIE inmore detail, an experiment was performed as shown in Table 1. TABLE 1Electron Carrier Contact Concen- Sam- Method of Forming Electrode (BackResistance tration ple Surface Treatment Condition) (Ω cm²) (cm⁻³⁾ 1Polishing Back Surface of GaN Substrate 20 2.0 × 10¹⁶ →Formation ofn-Side Electrode 2 Polishing Back Surface of GaN Substrate 0.1 5.0 ×10¹⁶ → Hydrochloric Acid Treatment → Formation of n-Side Electrode 3Polishing Back Surface of GaN Substrate 0.05 1.0 × 10¹⁷ →Etching by 0.5μm by RIE (Cl₂ Gas)→ Formation of n-Side Electrode 4 Polishing BackSurface of GaN Substrate 7.0 × 10⁻⁴ 7.1 × 10¹⁷ →Etching by 1 μm by RIE(Cl₂ Gas)→ Formation of n-Side Electrode 5 Polishing Back Surface of GaNSubstrate 3.0 × 10⁻⁴ 1.7 × 10¹⁸ →Etching by 1 μm by RIE (Cl₂ Gas + BCl₃Gas)→Formation of n-Side Electrode 6 Polishing Back Surface of GaNSubstrate 2.0 × 10⁻⁴ 2.5 × 10¹⁸ →Etching by 1 μm by RIE (Cl₂ Gas + BCl₃Gas) → Hydrochloric Acid Treatment → Formation of n-Side Electrode 7Polishing Back Surface of GaN Substrate 1.0 × 10⁻⁵ 5.0 × 10¹⁸ →Etchingby 1 μm by RIE (Cl₂ Gas + BCl₃ Gas) → Hydrochloric Acid Treatment →Formation of n-Side Electrode → Heat Treatment

[0063] Referring to Table 1, various nitrogen face (back surface)treatments were performed on samples 1 to 7 consisting of n-type GaNsubstrates having a wurtzite structure, for thereafter measuringelectron carrier concentrations in the vicinity of the back surfaces ofthe n-type GaN substrates with an electrochemical V-C measuredconcentration profiler. After this measurement of the electron carrierconcentrations, n-side electrodes were formed on the back surfaces ofthe n-type GaN substrates of the samples 1 to 7 for measuring contactresistance values between the n-type GaN substrates and the n-sideelectrodes by the TLM method.

[0064] The n-side electrodes of the samples 1 to 7 were formed by Alfilms, Si films, Ni films and Au films, similarly to the n-sideelectrode 8 according to the aforementioned embodiment. Substratepolishing, etching by RIE and hydrochloric acid treatment were performedunder conditions similar to those employed in the aforementionedembodiment. The sample 6 was prepared through the fabrication processaccording to the aforementioned embodiment.

[0065] In the inventive samples 3 to 7 prepared by etching the backsurfaces of the n-type GaN substrates by RIE, contact resistance valueswere remarkably reduced as compared with the sample 1 prepared by amethod similar to the prior art. More specifically, the sample 1exhibited contact resistance of 20 Ωcm², while the inventive samples 3to 7 exhibited contact resistance of not more than 0.05 Ωcm²,conceivably for the following reason: In the inventive samples 3 to 7,regions including defects resulting from mechanical polishing in thevicinity of the back surfaces of the n-type GaN substrates wereconceivably removed by RIE. Therefore, the electron carrierconcentrations were inhibited from reduction resulting from defects inthe vicinity of the back surfaces of the n-type GaN substrates in theinventive samples 3 to 7.

[0066] Further, the inventive samples 3 to 7 exhibited higher electroncarrier concentrations in the vicinity of the back surfaces of then-type GaN substrates as compared with the sample 1 corresponding to theprior art. More specifically, the sample 1 corresponding to the priorart exhibited an electron carrier concentration of 2.0×10¹⁶ cm⁻³, whilethe inventive samples 3 to 7 exhibited electron carrier concentrationsof at least 1.0×10¹⁷ cm⁻³.

[0067] In the sample 4 prepared by removing the back surface of then-type GaN substrate by a thickness of about 1 μm by RIE with Cl₂ gas,it was possible to attain lower contact resistance than the sample 3prepared by removing the back surface of the n-type GaN substrate by athickness of about 0.5 μm by RIE with Cl₂ gas. This is conceivablybecause it was not possible to sufficiently remove the region includingdefects resulting from mechanical polishing in the vicinity of the backsurface of the n-type GaN substrate by removing the back surface of then-type GaN substrate by the thickness of about 0.5 μm. When defect(dislocation) densities of the back surfaces of the n-type GaNsubstrates were measured by TEM analysis in these samples, the sample 3exhibited a defect density of 1×10⁹ cm⁻². In the sample 4, on the otherhand, no defects were observed in the field of view and the defectdensity was not more than 1×10⁶ cm⁻². Thus, it is preferable to removethe back surface of the n-type GaN substrate by a thickness of at leastabout 1.0 μm by RIE.

[0068] In the sample 5 subjected to RIE with Cl₂ gas and BCl₃ gas,contact resistance was further reduced as compared with the sample 4prepared by etching the back surface of the n-type GaN substrate by RIEwith only Cl₂ gas.

[0069] In the samples 6 and 7, corresponding to the aforementionedembodiment, prepared by etching the back surfaces of the n-type GaNsubstrates by RIE with Cl₂ gas and BCl₃ gas and thereafter performinghydrochloric acid treatment and the sample 7 further heat-treated in anitrogen atmosphere of 500° C. for 10 minutes, it was possible to obtainlower contact resistance values as compared with the sample 5 subjectedto neither hydrochloric acid treatment nor heat treatment. Comparing thesamples 6 and 7 with each other, it has been proved that the contactresistance between the n-type GaN substrate and the n-side electrode canbe further reduced and the electron carrier concentration in thevicinity of the back surface of the n-type GaN substrate can be furtherimproved by heat treatment.

[0070] In the sample 2 dipped in an HCl solution of 10% in concentrationfor about 10 minutes (hydrochloric acid treatment) without RIE, it waspossible to obtain lower contact resistance than the sample 1corresponding to the prior art subjected to no hydrochloric acidtreatment. More specifically, the sample 1 exhibited contact resistanceof 20 Ωcm2, while the sample 2 exhibited contact resistance of 0.1 Ωcm².This is conceivably because the back surface of the n-type GaN substratewas cleaned by the hydrochloric acid treatment.

[0071] If oxygen is employed as the n-type dopant for the n-type GaNsubstrate, crystallinity is reduced when the oxygen dose is increased toincrease the carrier concentration in order to reduce the contactresistance. According to the present invention, however, the contactresistance can be reduced also with the oxygen dose (substrate carrierconcentration: 5×10¹⁸ cm⁻³) for the n-type GaN substrate 1 according tothe aforementioned embodiment.

[0072] Although the present invention has been described and illustratedin detail, it is clearly understood that the same is by way ofillustration and example only and is not to be taken by way oflimitation, the spirit and scope of the present invention being limitedonly by the terms of the appended claims.

[0073] For example, while the above embodiment has been described withreference to the case of forming a nitride-based semiconductor laserdevice with the n-type GaN substrate 1, the present invention is notrestricted to this but an n-type nitride-based semiconductor substrateor a nitride-based semiconductor layer having a wurtzite structure mayalternatively be employed. For example, a nitride-based semiconductorsubstrate or a nitride-based semiconductor layer consisting of BN (boronnitride), AlN (aluminum nitride), InN (indium nitride) or TlN (thalliumnitride) is conceivable. The nitride-based semiconductor substrate orthe nitride-based semiconductor layer may consist of ternary orquaternary nitride-based semiconductor thereof.

[0074] While the back surface (nitrogen face) of the n-type GaNsubstrate 1 is etched by RIE in the aforementioned embodiment, thepresent invention is not restricted to this but other dry etching mayalternatively be employed. For example, reactive ion beam etching,radical etching, or plasma etching may be employed.

[0075] While the back surface (nitrogen face) of the n-type GaNsubstrate 1 is etched by RIE with Cl₂ gas and BCl₃ gas in theaforementioned embodiment, the present invention is not restricted tothis but other etching gases may alternatively be employed. For example,a gas mixture of Cl₂ and SiCl₄, a gas mixture of Cl₂ and CF₄ or Cl₂ gasmay be employed.

[0076] While the nitride-based semiconductor laser device structure 20is dipped in the HCl solution (hydrochloric acid treatment) after theetching by RIE thereby removing the chlorine-based residues adhering tothe back surface of the n-type GaN substrate 1 in the aforementionedembodiment, the present invention is not restricted to this but thenitride-based semiconductor layer device structure 20 may alternativelybe dipped in another solution containing at least one of chlorine,fluorine, bromine, iodine, sulfur and ammonia.

[0077] While the back surface (nitrogen face) of the n-type GaNsubstrate 1 is mechanically polished after growing the layers 2 to 6 onthe upper surface (Ga face) of the n-type GaN substrate 1 in theaforementioned embodiment, the present invention is not restricted tothis but the back surface (nitrogen face) of the n-type GaN substrate 1may alternatively be previously mechanically polished to a prescribedthickness for thereafter forming the layers 2 to 6 on the upper surface(Ga face) of the n-type GaN substrate 1. Further alternatively, thenitrogen face of the n-type GaN substrate 1 may not be mechanicallypolished.

[0078] While the n-type dopant and the p-type dopant for the layers 2 to6 are prepared from Si and Mg respectively in the aforementionedembodiment, the present invention is not restricted to this but anothern- or p-type dopant may alternatively be employed. For example, Se, Geor the like may be employed as the n-type dopant. Further, Be or Zn maybe employed as the p-type dopant.

[0079] While the layers 2 to 6 are formed on the n-type GaN substrate 1by atmospheric pressure MOCVD in the aforementioned embodiment, thepresent invention is not restricted to this but the layers 2 to 6 mayalternatively be formed by another growth method. For example, thelayers 2 to 6 may be formed by low-pressure MOCVD.

[0080] While the n-type buffer layer 2 is formed on the n-type GaNsubstrate 1 in the aforementioned embodiment, the present invention isnot restricted to this but no n-type buffer layer 2 may be formed. Inthis case, the fabrication process can be simplified although the layers3 to 6 are slightly reduced in crystallinity.

[0081] While the n-side electrode 8 is formed by the Al, Si, Ni and Aufilms in the aforementioned-embodiment, the present invention is notrestricted to this but the n-side electrode 8 may alternatively beformed by another electrode structure such as that consisting of a Tifilm having a thickness of 10 nm and an Al film having a thickness of500 nm, an Al film having a thickness of 6 nm, an Ni film having athickness of 10 nm and an Au film having a thickness of 300 nm or anAlSi film having a thickness of 10 nm, a Zn film having a thickness of300 nm and an Au film having a thickness of 100 nm, for example.

[0082] While a ridge structure is employed as a current narrowingstructure or a transverse light confinement structure in theaforementioned embodiment, the present invention is not restricted tothis but a current may alternatively be narrowed by an embeddedstructure employing a high-resistance blocking layer or an n-typeblocking layer. Further alternatively, a light absorption layer may beformed by ion implantation or the like as a current narrowing layer or atransverse light confinement structure.

[0083] While the present invention is applied to a nitride-basedsemiconductor laser device in the aforementioned embodiment, the presentinvention is not restricted to this but may be applied to asemiconductor device employing an n-type nitride-based semiconductorlayer or a nitride-based semiconductor substrate having a wurtzitestructure. For example, the present invention may be applied to a MESFET(metal semiconductor field-effect transistor), a HEMT (high electronmobility transistor), a light-emitting diode (LED) device or a VCSEL(vertical cavity surface emitting laser) device requiring surfaceflatness, for example.

[0084] While the p- and n-side electrodes 7 and 8 have prescribedthicknesses in the aforementioned embodiment, the present invention isnot restricted to this but the p- and n-side electrodes 7 and 8 mayalternatively have other thicknesses. For example, the electrodes 7 and8 may be reduced in thickness to have translucency, for employing thesemiconductor laser device as a VCSEL device or an LED device. Inparticular, the contact resistance of the n-side electrode 8 can besufficiently reduced according to the present invention also when then-side electrode 8 is formed with a small thickness to havetranslucency.

[0085] While the back surface (nitrogen face) of the n-type GaNsubstrate 1 is dry-etched by RIE in the aforementioned embodiment, thepresent invention is not restricted to this but the back surface(nitrogen face) of the n-type GaN substrate 1 may alternatively bewet-etched. In this case, aqua regia, KOH or K₂S₂O₈ is employed as thewet etchant. For example, the nitrogen face forming the back surface ofthe n-type GaN substrate 1 may be wet-etched under the room temperaturewith KOH of 0.1 mol in concentration. When the temperature is increasedto about 120° C. in this case, the etching rate can be increased toabout 10 times as compared with that under the room temperature.

[0086] While the back surface, consisting of the nitrogen face, of then-type GaN substrate 1 is dry-etched by RIE in the aforementionedembodiment, the present invention is not restricted to this but the backsurface of the n-type GaN substrate 1 may alternatively be wet-etchedwhen the back surface consists of a Ga face. In this case, aqua regia,KOH or K₂S₂O₈ is employed as the wet etchant. For example, the Ga faceforming the back surface of the n-type GaN substrate 1 may be wet-etchedin KOH of 0.1 mol in concentration with a mercury lamp of 365 nm underthe room temperature. When the temperature is increased to about 120° C.in this case, the etching rate can be increased to about 10 times ascompared with that under the room temperature.

[0087] While the n-type GaN just substrate 1 having the back surfaceentirely formed by a nitrogen face is employed in the aforementionedembodiment, the present invention is not restricted to this but ann-type GaN misoriented substrate having a back surface partiallyincluding a Ga face may alternatively be employed. Such back surface ofthe n-type GaN Disoriented substrate is also included in the nitrogenface according to the present invention.

What is claimed is:
 1. A method of fabricating a nitride-basedsemiconductor device, comprising steps of: etching the back surface of afirst semiconductor layer consisting of either an n-type nitride-basedsemiconductor layer or a nitride-based semiconductor substrate having awurtzite structure; and thereafter forming an n-side electrode on saidetched back surface of said first semiconductor layer.
 2. The method offabricating a nitride-based semiconductor device according to claim 1,wherein the back surface of said first semiconductor layer includes anitrogen face of said first semiconductor layer.
 3. The method offabricating a nitride-based semiconductor device according to claim 1,wherein said etching step includes a step of etching the back surface ofsaid first semiconductor layer by dry etching.
 4. The method offabricating a nitride-based semiconductor device according to claim 3,wherein said step of etching the back surface of said firstsemiconductor layer by dry etching includes a step of etching the backsurface of said first semiconductor layer by reactive ion etching withCl₂ gas and BCl₃ gas.
 5. The method of fabricating a nitride-basedsemiconductor device according to claim 4, wherein the ratio of the flowrate of BCl₃ gas to the flow rate of Cl₂ gas in said step of etching theback surface of said first semiconductor layer by said reactive ionetching is at least 30% and not more than 70%.
 6. The method offabricating a nitride-based semiconductor device according to claim 3,wherein the etching depth and the etching time in said step of etchingthe back surface of said first semiconductor layer by said dry etchingare proportional to each other.
 7. The method of fabricating anitride-based semiconductor device according to claim 1, wherein saidetching step includes a step of etching the back surface of said firstsemiconductor layer thereby converting the back surface of said firstsemiconductor layer to a mirror surface.
 8. The method of fabricating anitride-based semiconductor device according to claim 1, furthercomprising a step of performing heat treatment after said step offorming said n-side electrode.
 9. The method of fabricating anitride-based semiconductor device according to claim 1, wherein saidetching step includes a step of etching the back surface of said firstsemiconductor layer by a thickness of at least about 1 μm.
 10. Themethod of fabricating a nitride-based semiconductor device according toclaim 1, wherein said first semiconductor layer includes said n-typenitride-based semiconductor layer or said nitride-based semiconductorsubstrate consisting of at least one material selected from a groupconsisting of GaN, BN, AlN, InN and TlN.
 11. The method of fabricating anitride-based. semiconductor device according to claim 1, wherein saidn-side electrode includes an Al film.
 12. The method of fabricating anitride-based semiconductor device according to claim 1, wherein saidnitride-based semiconductor device is a nitride-based semiconductorlight-emitting device.
 13. The method of fabricating a nitride-basedsemiconductor device according to claim 1, further comprising a step ofdipping a nitrogen face of said etched first semiconductor layer in asolution containing at least one of chlorine, fluorine, bromine, iodine,sulfur and ammonium in advance of said step of forming said n-sideelectrode.
 14. The method of fabricating a nitride-based semiconductordevice according to claim 13, further comprising a step of performinghydrochloric acid treatment on the back surface of said firstsemiconductor layer with an HCl solution in advance of said step offorming said n-side electrode.
 15. The method of fabricating anitride-based semiconductor device according to any of claims 1 to 8,further comprising a step of polishing the back surface of said firstsemiconductor layer in advance of said etching step.
 16. The method offabricating a nitride-based semiconductor device according to claim 1,wherein said etching step includes a step of etching the back surface ofsaid first semiconductor layer by wet etching.
 17. The method offabricating a nitride-based semiconductor device according to claim 16,wherein said step of etching the back surface of said firstsemiconductor layer by wet etching includes a step of etching the backsurface of said first semiconductor layer with at least one etchantselected from a group consisting of aqua regia, KOH and K₂S₂O_(8.) 18.The method of fabricating a nitride-based semiconductor device accordingto claim 16, wherein said step of etching the back surface of said firstsemiconductor layer by wet etching includes a step of etching the backsurface of said first semiconductor layer while increasing thetemperature to about 120° C.
 19. A method of fabricating a nitride-basedsemiconductor device, comprising steps of: etching a nitrogen face of afirst semiconductor layer consisting of either an n-type nitride-basedsemiconductor layer or a nitride-based semiconductor substrate having awurtzite structure by dry etching; and thereafter forming an n-sideelectrode on said etched nitrogen face of said first semiconductorlayer.
 20. A nitride-based semiconductor device formed through steps of:etching the back surface of a first semiconductor layer consisting ofeither an n-type nitride-based semiconductor layer or a nitride-basedsemiconductor substrate having a wurtzite structure; and thereafterforming an n-side electrode on said etched back surface of said firstsemiconductor layer.
 21. A nitride-based semiconductor devicecomprising: a first semiconductor layer consisting of either an n-typenitride-based semiconductor layer or a nitride-based semiconductorsubstrate having a wurtzite structure; and an n-side electrode formed onthe back surface of said first semiconductor layer, wherein contactresistance between said n-side electrode and said first semiconductorlayer is not more than 0.05 Ωcm².
 22. The nitride-based semiconductordevice according to claim 21, wherein an electron carrier concentrationis at least 1×10¹⁷ cm⁻³ in the vicinity of the interface between saidfirst semiconductor layer and said n-side electrode.
 23. Thenitride-based semiconductor device according to claim 21, wherein adislocation density is not more than 1×10⁹ cm⁻² in the vicinity of theinterface between said first semiconductor layer and said n-sideelectrode.
 24. The nitride-based semiconductor device according to claim21, wherein the back surface of said first semiconductor layer includesa nitrogen face of said first semiconductor layer.
 25. The nitride-basedsemiconductor device according to claim 21, wherein said firstsemiconductor layer includes said n-type nitride-based semiconductorlayer or said nitride-based semiconductor substrate consisting of atleast one material selected from a group consisting of GaN, BN, AlN, InNand TlN.
 26. The nitride-based semiconductor device according to claim21, wherein said n-side electrode includes an Al film.
 27. Thenitride-based semiconductor device according to claim 21, wherein saidnitride-based semiconductor device is a nitride-based semiconductorlight-emitting device.